1. Field of the Invention
The present general inventive concept relates to a developing cartridge, and more particularly, to a developing cartridge with an improved power transmission structure that transmits power to a developing unit so that the developing unit can keep a developing gap constant.
2. Description of the Related Art
An image forming apparatus of an electrophotography type forms an image on a print medium using toner in a sequential order of static electricity erasing, charging, exposing, developing, transferring, and fixing operations. An example of the image forming apparatus may include a copy machine, a laser printer and so on. Such an electrophotography-typed image forming apparatus has a developing cartridge that is detachably provided in a body of the image forming apparatus and stores toner.
As illustrated in FIGS. 1 to 3, a conventional developing cartridge 1 includes a photosensitive unit 30 including a photosensitive drum 33, and a developing unit 20 including a developing roller 23.
The photosensitive unit 30 further includes a cleaning blade 35 that cleans out used toner adhering to a surface of the photosensitive drum 33 after an image is transferred into a print medium, and a charging roller 31 that contacts and charges the photosensitive drum 33. A shaft 33a of the photosensitive drum 33 is inserted and rotatably supported in shaft holes 43 of side frames 40 that are provided in opposite sides of the developing cartridge 1. A photosensitive body driven coupling (not illustrated) for receiving power from an internal main driver of an image forming apparatus (not illustrated) is provided at one end portion of the shaft 33a when the developing cartridge 1 is mounted in the image forming apparatus. The main driver of the image forming apparatus includes a photosensitive body driving coupling (not illustrated) that engages with the photosensitive body driven coupling in the direction of the shaft 33a. The photosensitive body driving coupling is driven by a driving motor (not illustrated) provided in the body of the image forming apparatus.
The developing unit 20 develops the photosensitive drum 33 with toner stored in a unit casing 21. The unit casing 21 rotatably supports an agitator 29 that feeds the toner to a supplying roller 25, the supplying roller 25 that frictionally charges the toner fed from the agitator 29, and the developing roller 23 that develops the photosensitive drum 33 with the frictionally-charged toner using an electric force. A doctor blade 27 is illustrated in the developing unit 20 to regulate a thickness of the toner attached to the developing roller 23.
In addition, the developing unit 20 includes an agitator gear 29a connected to a shaft of the agitator 29, and transmission gears 26 and 28 that are interposed between the agitator gear 29a and a driven coupling 67, which will be described later, and transmit a rotatory power of the driven coupling 67 to the agitator 29a. A support plate 24 that rotatably supports the transmission gears 26 and 28 and the driven coupling 67 is joined to the unit casing 21 in such a manner that an inter axial distance between the transmission gears 26 and 28 and the driven coupling 67 can keep constant.
On the other hand, the developing unit 20 is provided to pivotally rotate around a hinge shaft 10. That is, the hinge shaft 10 is inserted in the support plate 24 and is rotatably supported by the side frames 40. In addition, the side frames 40 are respectively connected to projections 21a of the unit casing 21 through elastic members in order to restrain the developing unit 20 from excessively rotating around the hinge shaft 10. Accordingly, the developing unit 20 rotates around the hinge shaft 10 to some proper extent.
In addition, as illustrated in FIGS. 2 and 3, the conventional developing cartridge 1 has a power transmission structure to rotate a rotating body of the developing unit 20. The driven coupling 67 illustrated in FIG. 3 is joined to one end of a shaft (23a of FIG. 1) of the developing roller 23. The driven coupling 67 has an engaging member 67b which is formed on an outer circumference surface of the driven coupling 67 and is engaged with the transmission gear 26.
In addition, a driving coupling 63, which is joined to the driven coupling 67 in the direction of the shaft (23a of FIG. 1) of the developing roller 23, is rotatably supported to the right side frame (not illustrated) facing the left side frame 40 illustrated in FIG. 2.
The driving coupling 63 and the driven coupling 67 have two driving projections 63a and two driven projections 67a, respectively, which project toward each other. The driving projections 63a and the driven projections 67a are spaced from each other in a circumferential direction, and are inserted in projection grooves 65a of a rotating disk 65. The rotating disk 65 is joined to and integrally rotates with the driven coupling 67.
The driving coupling 63 receives the rotatory power from the internal main driver of the image forming apparatus when the developing cartridge 1 is mounted in the image forming apparatus. The power transmission system uses the driving coupling 63 and the driven coupling 67 to transmit a rotatory driving force to the developing roller 23 uniformly when the developing unit 20 rotates.
In addition, the developing unit 20 and the photosensitive unit 30 receive power from the main driver through different power transmission structures.
On the other hand, as illustrated in FIGS. 4A and 4B, as the developing roller 23 rotates in an opposite direction (counterclockwise direction indicated by D in FIG. 1) to a rotation direction (clockwise direction indicated by C in FIG. 1) of the photosensitive roller 23, the driving coupling 63 is driven in the counterclockwise direction. Accordingly, the driving projections 63a also rotate in the counterclockwise direction, thereby rotating the rotating disk 65 and the driven coupling 67 in the same direction.
As illustrated in FIG. 4A, however, when first and second driving projections 63a1 and 63a2 are disposed in a radical direction of the hinge shaft 10, a moment arm r1 of a first driving projection 63a1 centering around the hinge shaft 10 of the developing unit 20 is shorter than a moment arm r2 of a second driving projection 63a2, and a force F applied to the rotating disk 65 by the first driving projection 63a1 is nearly equal in magnitude to a force F applied to the rotating disk 65 by the second driving projection 63a2. Accordingly, a rotation moment M2, which is caused by the second driving projection 63a2, rotating around the hinge shaft 10 of the developing unit 20 is larger than a rotation moment M1, which is caused by the first driving projection 63a1, rotating around the hinge shaft 10 of the developing unit 20. Accordingly, the developing unit 20 becomes away from the photosensitive drum 33 by a resultant rotation moment M0 which is the sum of rotation moments M1 and M2.
On the other hand, as illustrated in FIG. 4B, when the first and second driving projections 63a1 and 63a2 are disposed opposite to each other with respect to a radical direction of the hinge shaft 10, that is, when the moment arm r1 of the first driving projection 63a1 rotating around the hinge shaft 10 is equal to the moment arm r2 of the second driving projection 63a2, the rotation moment M2 which is caused by the second driving projection 63a2 is in equilibrium with the rotation moment M1 which is caused by the first driving projection 63a1, and accordingly, the resultant rotation moment M0 applied to the developing unit 20 becomes zero.
FIG. 5 illustrates arrangement of the resultant rotation moments M0 of FIGS. 4A and 4B, that is, a change of the resultant rotation moments M0 with respect to a time axis. As illustrated in FIG. 5, when the driving coupling 63 rotates one time, the resultant rotation moment M0 reaches the maximum value two times, and always has (+) values except when the moment arms r1 and r2 are equal to each other as illustrated in FIG. 4B. Here, a rotation moment in a direction in which the developing unit 20 is away from the photosensitive drum 33 is defined to have a (+) value.
On the other hand, a method of developing the photosensitive drum 33 may include a contact method of the developing the photosensitive drum 33 at a developing nip at which the developing roller 23 contacts the outer circumference surface of the photosensitive drum 33 and a non-contact method of developing the photosensitive drum 33 through a developing gap which is interposed between the developing roller 23 and the photosensitive drum 33 to make them distanced apart from each other.
However, in the conventional developing cartridge 1, since the resultant rotation moment M0 always has the (+) values, the developing roller 23 suffers from a rotation moment by which the developing roller 23 is spaced from the photosensitive drum 33. Accordingly, the developing nip or the developing gap varies with respect to the time axis. Thus, concentration of an image periodically varies in a longitudinal direction of a print medium, which results in deterioration of print quality.
Particularly, in a case of the non-contact method, it can be seen from FIG. 6 that aged toner stored in the unit casing 21 responds more sensitively to change of the developing gap than fresh toner, which leads to a rapid change of concentration of an image. Accordingly, since aged toner increases with increase of use time of the developing cartridge, print quality is more deteriorated.